The present invention provides articles formed from dense polymer membranes and films, which articles spontaneously inflate/expand (or deflate/contract) under the effects of osmotic pressure. By varying the initial volume, molecular weight, solubility, concentration, and temperature of solutions contained within formed articles, the volume of the formed articles and the pressure within the formed articles can be precisely controlled. This invention is therefore useful for many medical, industrial, and consumer applications.
U.S. Pat. No. 4,157,085 discloses a surgically implantable device for expanding skin and mucous tissue. The device comprises a partially collapsed sealed envelope formed from a material that is permeable to extracellular body fluids, and a material which establishes an osmotic potential across the envelope wall. As bodily fluid crosses the membrane to fill the envelope, the envelope expands, thereby expanding the adjacent tissue. The envelope material, which ranges from 0.001 to 0.020 inches in thickness, can be formed from such polymers as cellulose acetate, crosslinked polyvinyl alcohol, polyurethane, nylon, ethylene vinyl acetate copolymer, polyolefin, polyvinyl chloride, and silicone rubber. The material used to establish the osmotic potential can be, for instance, sodium chloride, calcium chloride, potassium sulphate, sodium acetate, ammonium phosphate, magnesium succinate, glucose, alanine, vinyl pyrrolidone, carboxymethylcellulose, urea, xe2x80x9cblood by-productsxe2x80x9d, proteins, and/or dextran. The amount of expansion material used is selected depending on the desired speed of expansion of the envelope and the desired final volume thereof.
U.S. Pat. No. 5,005,591 points out that the device described in U.S. Pat. No. 4,157,085 has problems. One problem is in the use of silicone as the envelope material. In thicknesses that are adequate for product safety, silicone is relatively impermeable to extracellular water. In order to compensate for the required silicone thickness and allow for timely inflation of the tissue, the implant must contain an inordinately high amount of solute. Another, related problem is that rupture of the implanted envelope results in dispersion of highly concentrated solute into the surrounding tissues.
EP 0 784 987 discloses an inflatable prosthesis that contains a dehydrated substance which forms a gel when mixed with an aqueous solution. The dehydrated substance is a polyacrylamide, polyvinylpyrrolidone, hydroxypropyl methyl-cellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, polyethylene glycol, polylactic acid polyglycolic acid, hydrogel polyurethane, chrondotoin sulfate, hyaluronic acid, alginate, etc. The dehydrated substance is placed within an outer shell constructed from a polydimethyl siloxane, a polyurethane, a polyurethane/polyester copolymer, or another similar viscoelastic material. As shown in FIG. 3 of the publication, after the prosthesis is implanted into the body, an aqueous solution is added to the inner cavity thereof by a syringe. The addition of the aqueous solution combines with the dehydrated substance in the cavity to form a gel within the implant.
U.S. Pat. No. 5,756,632 claims an article comprising a hydrophilic gel with a water content of at least about 35% substantially enclosed within a membrane comprising a biocompatible, hydrophilic, segmented block polyurethane copolymer. The block copolymer comprises about 5 to 45 weight-% of at least one hard segment and about 95 to 55 weight-% of at least one soft segment. The soft segment of the block copolymer comprises at least one hydrophilic, hydrophobic, or amphipathic oligomer selected from the group consisting of aliphatic polyols, aliphatic polyamines, aromatic polyamines, and mixtures thereof. This patent also claims a method of permeating moleculesxe2x80x94such as glucose, angiotensin I, glucagon, insulin, aprotinin, albumin, IgG, and oxygenxe2x80x94while substantially preventing the passage of cells and particulate matter between two fluids, using a biocompatible, hydrophilic, segmented block polyurethane copolymer.
U.S. Pat. No. 6,264,695 discloses a spinal nucleus implant for replacement of a portion of nucleus pulposus tissue removed from a spinal disc. The implant is an anisotropically swellable, biomimetic xerogel plastic, having a negatively charged lubricious surface and having a two phase structure with a hydrophobic phase having high crystallinity and low water content and a hydrophilic phase having low crystallinity and high water content. The xerogel plastic is capable of rehydration and of osmotic movement of liquid therethrough in response to osmotic pressure change to thereby increase and decrease liquid content in its hydrated state. Example 1 of the patent molds an acrylonitrile/acrylic acid block copolymer into the shape of a nucleus pulposus, and indicates that when the molded article is implanted into a vertebrate spine, it will be partially hydrated by body fluids, swelling to the shape and tension required for its long-term function. The patent also discloses surgical implant procedures utilizing the disclosed spinal nucleus implant.
The present invention provides devices that are implantable in animal bodies. In one embodiment, the devices of the present invention include a solute that is encapsulated by a water-permeable polymer membrane. The membrane polymer will ideally be biocompatible, strong and flexible, abrasion resistant, biostable, and highly permeable to water while providing an effective barrier to the solute. The membrane should also be sterile and processable by thermoplastic or solution methods suitable for device fabrication. The solute should be water soluble, non-toxic, and biostable within the internal cavity of the device.
One specific application of the present invention is a self-inflating/shape-memory prosthetic nucleus for percutaneous insertion in a minimally invasive treatment of human spinal disc degeneration. Another embodiment of this invention is a method for treating a spine of a vertebrate which comprises inserting such a device into an area of the spine that contains a degenerated, damage, or missing disc and allowing the device to expand by permeation of aqueous fluid into a cavity containing the solute.
This invention includes the use of a water-permeable polymer (preferably biostable) in the form of a dense membrane with moderate to low water absorption (less than 50 weight-%, preferably less than 15 weight-%, of the combined weight of the polymer and the absorbed water) containing a dispersed polymeric or monomeric solute to fabricate a device or formed article. This invention provides a high strength, dense osmotic membrane containing a low molecular weight solute to produce high osmotic pressure.
In summary, this invention provides a device implantable in an animal (including human) body. The device comprises a water-absorbing solute encapsulated by a non-porous, water-permeable polymer membrane. The membrane has a moisture vapor transmission rate (MVTR) by ASTM Method E96BW greater than 100 g/square meter/day at 1 mil thickness, a dry tensile strength of greater than 500 psi, and a permeability to said solute of less than 10xe2x88x923 cm2/sec. In preferred embodiments of this invention, the MVTR minimum may be  greater than 1000,  greater than 2000,  greater than 5000, or  greater than 10,000 g/square meter/day at 1 mil thickness, the tensile strength may be  greater than 1000,  greater than 2000, or  greater than 5000 psi, and the permeability may be  less than 10xe2x88x924,  less than 10xe2x88x925, or  less than 10xe2x88x926 cm2/sec. The solute is capable of providing said device with a pressure of approximately 0.1 psi or more for a period of at least 24 hours. In preferred embodiments of this invention, the osmotic pressure that can be held may be  greater than 1,  greater than 10, or  greater than 100 psi and the osmotic pressure can be held at those levels for  greater than 1 week,  greater than 6 weeks, or  greater than 1 year.
In accordance with this invention, each of these parameters may be varied independently. In a particularly preferred device of this invention, the membrane has a moisture vapor transmission rate by ASTM Method E96BW greater than 2000 g/square meter/day at 1 mil thickness, a dry tensile strength of greater than 2000 psi, and a permeability to said solute of less than 10xe2x88x924 cm2/sec, and the solute is capable of providing the device with a pressure of approximately 100 psi or more for a period of at least six weeks.
In the devices of this invention, the membrane polymer is preferably a block copolymer having a polyurethane and/or polyurea hard segment and a soft segment selected from the group consisting of polyalkylene oxides, polyalkylene carbonates, polysiloxanes, and linear or branched hydrocarbons. The soft segment is most preferably a polyalkylene oxide soft segment. Devices made in accordance with this invention generally employ membranes having thicknesses within the range 1 to 16 mil.
For some applications, the membrane used to make the devices may have dispersed therein or covalently bound thereto a water-soluble additive, such as sodium chloride, glycerine, or polyethylene oxides. This permits the devices to decrease in permeability over time.
In the devices of this invention, the solute is most preferably a polyacrylamide having a nominal weight-average molecular weight of 1200-3000 Daltons. The solute is normally present in devices of this invention in amounts of 0.1-50 grams, preferably in amounts of 1-3 grams.
In one specific embodiment of this invention, the device is a self-inflating prosthetic spinal disc nucleus. In another specific embodiment, the device is a self-inflating spinal disc nucleus surrounded by a reinforced perimeter and attachment means, configured to form a complete spinal disc prosthesis. A method embodiment of this invention includes treating a spine of a vertebrate by inserting one of these devices into an area of the spine that contains a degenerated, damaged, or missing disc and allowing the device to expand by permeation of aqueous fluid into a cavity containing the solute.
Among contemplated variations of this invention are devices in which the solute comprises a blend of low molecular weight solute for quick inflation of the device and a high molecular weight solute to provide long term pressure and stability and devices in which the membrane decreases in permeability over time to permit rapid short term inflation and persistent long term avoidance of solute leakage.
This invention also provides a method for obtaining sustained maintenance of high pressure in a device implanted in an animal body, said method comprising the steps of: selecting a membrane to have a moisture vapor transmission rate by ASTM Method E96BW greater than 100 g/square meter/day at 1 mil thickness and a dry tensile strength of greater than 500 psi; selecting a solute to be capable of providing said device with a pressure of approximately 0.1 psi or more for a period of at least 24 hours; encapsulating said solute in said membrane to form said device; and implanting said device into said body, whereupon the solute in said device absorbs water from ambient aqueous fluids present in said body until it is inflated to a pressure of approximately 0.1 psi or more. All of the preferred device parameters specified above are relevant to this method. In an especially preferred embodiment, this method comprises the steps of: selecting a membrane to have a moisture vapor transmission rate by ASTM Method E96BW greater than 2000 g/square meter/day at 1 mil thickness and a dry tensile strength of greater that 2000 psi; selecting a solute to be capable of providing said device with a pressure of approximately 100 psi or more for a period of at least six weeks; encapsulating said solute in said membrane to form said device; and implanting said device into said body, whereupon the solute in said device absorbs water from ambient aqueous fluids present in said body until it is inflated to a pressure of approximately 100 psi or more. Inflation to the target pressure can, if desired, be accomplished within a period of 24 hours.
In a non-biomedical context, this invention provides a potable-liquid producing device comprising a water-absorbing solute encapsulated by a non-porous, water-permeable polymer membrane. As above, the membrane has a moisture vapor transmission rate by ASTM Method E96BW greater than 100 g/square meter/day at 1 mil thickness, a dry tensile strength of greater than 500 psi, and a permeability to said solute of less than 10xe2x88x923 cm2/sec, and the solute is capable of providing said device with a pressure of approximately 0.1 psi or more for a period of at least 24 hours. As above, each parameter is increased (or in the case of permeability, decreased) in preferred embodiments of the invention. The related method of desalinating and/or purifying water by immersing the device of in a body of water to be desalinated or purified is likewise an aspect of the present invention.